Welcome! You have a good looking truck there stcengine.com(stcsoshin) carry 4d56's they were selling them thru ebay for some time and have pretty good feedback i bought a diesel thru a company in texas but can't remember the name to be honest. There is a guy here that goes by the name plh that might be able to get you a tach or you can use a dakota digital converter to use your gas tach. 4k is also bit much to pay for a used 4d56.

btw i play bass guitar i currently own a 89 charvel with roto sound steve harris flats and a emg P bass pickups if i were to ever get a bass made it would have to have that charvel neck profile its just perfect.